1. Field of the Invention
The present invention relates to a hexagonal cell honeycomb structure body for use as a catalyst support capable of supporting catalysts in an exhaust gas purifying device mounted to an internal combustion engine for vehicles, and purifying an exhaust gas emitted from an internal combustion engine such as a diesel engine.
2. Description of the Related Art
Recently, various types of an improved honeycomb structure body have been proposed, each of which is comprised of a plurality of cells partitioned by cell walls. Each of the cell walls has a reduced thickness in order to rapidly activate catalysts supported therein. Such a honeycomb structure body has a light weight and a reduced heat capacity because of having the cell walls of a reduced thickness, namely, a thin thickness. The honeycomb structure body acts as a catalyst support capable of supporting catalysts used in an exhaust gas purifying device such as a diesel particulate filter (DPF), for a diesel engine mounted on a vehicle.
In order to enhance or improve the performance of purifying an exhaust gas, a hexagonal honeycomb structure body having hexagonal shaped cells (hereinafter, referred to as the “hexagonal cell honeycomb structure body”) is widely used because of being capable of uniformly supporting catalyst thereon, and allows the exhaust gas emitted from an internal combustion engine to be passing. The hexagonal cell honeycomb structure body having the above structure comprised of the hexagonal shaped cells can decrease its pressure loss because of allowing the exhaust gas to be smoothly passing therethrough.
However, the hexagonal cell honeycomb structure body as a catalyst support is usually accommodated or assembled in a cylindrical case to which a predetermined magnitude of pressure is applied from an outer peripheral side toward the inside of the hexagonal cell honeycomb structure body. Because the more the thickness of the cell walls in the hexagonal cell honeycomb structure body is decreased (for example, not more than 140 μm), the more the rigidity of the cell walls is decreased. This causes a problem to decrease a breaking strength (or a compressive fracture strength) of the hexagonal cell honeycomb structure body by the pressure applied from the outside of the hexagonal cell honeycomb structure body. (see the conventional patent document 1 described later)
In addition, it is generally difficult to avoid the generation of distortion in the cell walls during a shaping step, a drying step, and another step in the production process of the hexagonal cell honeycomb structure body. In particular, when the hexagonal cell honeycomb structure body is comprised of cell walls of not more than 140 μm thickness, there is a tendency to decrease its isostatic strength. This needs to take a measure to increase the isostatics strength.
Decreasing the breaking structure (or a compressive fracture strength) causes a serious problem in the hexagonal cell honeycomb structure body comprised of the hexagonal shaped cells rather than triangle shaped cells or square shaped cells.
There are various related art documents to increase or improve the strength of the entire of the hexagonal cell honeycomb structure body. For example, Japanese patent laid open publication No. JP 2007-196212 has disclosed a hexagonal cell honeycomb structure body which satisfies a relationship of Da/P≧0.13, where Da is a diameter of an inscribed circle which is inscribed through the injection of adjacent cell walls, and P denotes a cell pitch. This conventional structure can maintain the breaking strength even if the thickness of each of the cell walls in a hexagonal cell honeycomb structure body is thin, for example, not more than 140 μm.
Although the conventional technique disclosed in the above conventional patent document JP 2007-196212 has an adequate breaking strength as a mechanical strength, the junction area between adjacent cell walls is increased, the total weight of the hexagonal cell honeycomb structure body is thereby increased. This also increases its heat capacity because of increasing its total weight. This deteriorates the performance of rapidly activating catalysts supported on the cell walls with which cells are formed in the hexagonal cell honeycomb structure body, and thereby makes it difficult to adequately show the feature of rapidly activating the catalysts with the thin structure of the cell walls.